1. Field of the Invention
The present invention relates to a micro-electro-mechanical systems (MEMS) device and a method of making the same, and, more particularly, to a MEMS device with a lateral vent hole and a method of making the same.
2. Description of the Prior Art
MEMS devices include micromachines integrated with electronic microcircuits on substrates. Such devices may form, for example, microsensors or microactuators which operate based on, for example, electromagnetic, electrostrictive, thermoelectric, piezoelectric, or piezoresistive effects. MEMS devices have been formed on insulators or other substrates using micro-electronic techniques such as photolithography, vapor deposition, and etching. Recently, MEMS is fabricated using the same types of steps (such as the deposition of layers of material and the selective removal of the layers of material) that are used to fabricate conventional analog and digital complementary metal oxide semiconductor (CMOS) circuits.
The recent ability to seal micro-machined meshes has lead to the fabrication of microphones and microspeakers. A sealed mesh can function as a movable plate of a variable capacitor, and therefore can operate as a microspeaker or microphone. For a sealed mesh to operate as a microspeaker or microphone, the device needs to be able to push air to create a soundwave just as its larger counterparts must push air to create soundwaves. In the case of a microspeaker or microphone, if the chamber beneath the sealed mesh does not have a vent or other opening to ambient, movement of the sealed mesh inward is inhibited by the inability to compress the air in the chamber while movement of the mesh outward is inhibited by formation of a vacuum. Thus it is necessary to form a vent in the chamber.
Currently, such vents are formed by boring through the silicon substrate from the rear. For example, a method of making a MEMS device is disclosed in U.S. Pat. No. 6,936,524 that comprises some steps as shown in FIGS. 1 and 2. FIG. 1 shows the formation of vent holes after the mesh has been released and the pilot openings expanded. As shown in FIG. 1, a first dielectric layer 14, a first metal layer 16, a second dielectric layer 20, a second metal layer 22, a third dielectric layer 26, a third metal layer 28, a top dielectric layer 32, and a photo-resist layer 38 are stacked on the right surface of the silicon substrate 12. The first metal layer 16 is patterned to allow a portion thereof to form a structure of micro-machined mesh metal 18. The second and the third metal layers 22 and 28 each have an opening above the micro-machined mesh metal 18 to expose the micro-machined mesh metal 18. The photo-resist layer 38 covers the above of the third metal layer 28 to protect the portion not to be etched. The reverse surface of the silicon substrate 12 is adhered to a first carrier wafer 36 through an adhesive 34. Thus, a deep reactive-ion etching (DRIE) process, and subsequently reactive-ion etching (RIE) process, inductively coupled plasma reactive ion etching process, or XeF2 etching process 24 are performed on the right surface of the silicon substrate 12 to partially etch the silicon substrate 12 and to release the micro-machined mesh metal 18 to form vent holes 40. FIG. 2 shows another example. After the right surface of the silicon substrate 12 is protected by a protection layer 45 and adhered to a second carrier wafer 44 through an adhesive 42, an RIE or a DRIE process 48 is performed on the silicon substrate 12 using a photo-resist mask 46 from the reverse surface of the silicon substrate 12. However, the silicon substrate typically has a thickness of about 700 microns, and it may still remain more than 300 microns even after certain polishing steps are carried out during the manufacturing process. It would be a tedious process to etch though the substrate no matter from the rear or the front.
Therefore, there is still a need for a novel MEMS device structure and the making method to conveniently making such devices.